8 ANNALS NIJW YORK ACADEMY OF SCIENCES 



Cactocrinus opusculus (Mall) 

 Plate I, fig. 4; plate IV, figs. 3, 4, 4o, 4b, 4c, 4d 



I860.' A cHnocrinut opugrulus Hftll. Suppl. (Jeol. Kept. Iowa, see description of 



pi. L'. 

 1897. Cactocrinus opunculus Waclismuth and Springer, Nortli American Crinoi- 



dea Camerata, p. 607, pi. 56, figs. 5a, r}b. 



The calyx of this species bears a close resemblance to that of Cacto- 

 crinus reticulatus, except in the greater number of plates necessary for 

 the support of six anns to the ray, which is the normal number for C/. 

 opusculus. The surface of the calyx is ornamented by a single node at 

 the center of each plate with connecting carinse^ as in C. reticulatus. 



At the base, the arms are, as usual in the genus, cylindrical and smooth, 

 but they soon become flattened laterally, as in C. prohoscidalis, and in 

 retarded specimens bear the strong transverse ridge on the arm plates 

 characteristic of that species. One specimen retains the C. prohoscidalis 

 type of ornament for 35 or 30 plates, while in accelerated individuals this 

 condition is represented by only 2 or 3 plates, or may be absent altogether. 

 In average individuals, at a distance from the base varying from the 

 tenth to the twentieth plate, a slight angulation appears on the arm near 

 the median line. This slight elevation increases in size on succeeding 

 plates until it foruis a distinct node. At the same time, it recedes far- 

 ther and farther from the median line. The lateral node continues to 

 increase in prominence until it Ixicomcs a distinct sj)iiu', and accoTn})any- 

 ing this change in the node, and partly in consequence of it, the form of 

 the arm changes until it is strongly flattened dorso-ventrally. Mean- 

 while, another row of nodes has come in close to the median line on each 

 side, as in Cactocrinus reticulatus. At the highest point observable the 

 arm is strongly flattened dorso-ventrally with a row of spines along each 

 lateral margin and a row of nodes on each side of the median line. 



From the lime the angulation appears until it develops into a distinct 

 node, the surface of the plates is distinctly corrugated, although the 

 strength of the corrugation varies greatly in different specimens. The 

 specimen figured in Plate 1. fig. 4. and Plate IV, fig. 3, is a highly accel- 

 erated individual showing all the characters at the acme of their develop- 

 ment. 



Horizon and locality : liowor Turlington limestone, Burlington, Iowa. No. 

 523, Museum of Comparative Zoology collection. 



